Blown Head Gasket from Coolant Passage Corrosion - Replacement Head Install

Engine was overheating and pushing coolant out of the reservoir, indicating a blown head gasket on a 7MGE NA-T.

Severe corrosion in the exhaust-side coolant passages of the cylinder head, particularly at cylinder #5, had eaten through to the head gasket copper sealing material. This allowed combustion gases to leak past the gasket into the cooling system, especially under boost. The corrosion was likely caused by electrolysis from old or acidic coolant (coolant pH dropping to 7 or below).

High confidence — Original poster confirmed this resolved the issue